1. Field of the Invention
This invention relates generally to the field of performing parallel deformations upon sheets of material. More specifically, it relates to a positioning device used to measure the distance between deformation tools. Even more specifically, it relates to a positioning device used to measure the distance between the cutting edges of two knives in a can blank slitter machine.
2. Description of the Related Technology
To make the body of a three piece can, a large sheet of metal is cut into smaller can blanks by a can blank slitter, which includes a first series of knives that are arranged to make parallel slits in a sheet of metal. This results in the sheet of metal being cut into strips. A can blank slitter also has a second series of knives that are arranged to make parallel slits across the strips of metal. The resulting rectangular pieces of sheet metal are the can blanks that eventually become the body of a three piece can.
Typically, the knives in a can blank slitter are metal disks mounted to a shaft. The knives are arranged such that their central axes are coextensive with each other and the shaft's central axis. The cutting edge of a knife is one of the two edges of its cylindrical, perimeter surface.
The accurate and precise setting of knives in a can blank slitter machine is crucial to can quality. Can blanks must be cut to within a tolerance of 5/1000 of an inch for the length and 2/1000 of an inch for the height to have proper welds on the final can. Improper welds can result in gaps in a can or a burst weld seam.
In the past, knives have been set using a "GO-NO GO" gage measuring the distance between the knives and a commercial slitter set-up gage. Both of these methods have their short comings.
One shortcoming of the "GO-NO GO" gage is that it does not measure the distance between the two cutting edges. The cutting edges of all the knives in a series are either the right edge or the left edge, not mixed. Since the "GO-NO GO" gages measure the distance between a cutting edge and a non-cutting edge, the distance between the cutting edges is not being directly measured. Since the distance is not being directly measured, the possibility of inaccurate and imprecise knife placement results.
Another shortcoming of the "GO-NO GO" gage is that the gage assumes the disks are the same thickness. The "GO-NO GO" gage operates on the principle that the distance between two cutting edges is the distance between the facing surfaces of the knives plus the thickness of a knife. However, in actual use the knives are ground down, changing their thickness. The "GO-NO GO" gage does not allow for variations in knife thickness. This results in a lesser degree of control of the setting of the knives, resulting in inaccurate and imprecise can blanks being cut.
Two shortcomings of a commercial slitter set-up gage, such as "MAWAG" Digital Roller Cutter Measurement System, are its technological complexity and expense. The distance between the knives is measured by moving a pawl from one knife to the other knife, measuring the distance the pawl travels, generating an electrical signal representing that distance. The results of the measurement are displayed electronically. The result of using this technology is a system costing thousands of dollars.
It is clear that there has existed a long and unfulfilled need in the prior art for an improved gage to position can blank slitter knives that is accurate, precise, not technologically complex, and inexpensive.